1. Field of the Invention
The present invention relates to an apparatus and method for surface treatment of a substrate, which is useful for removing contaminants, such as an organic material, an oxide on a metal surface, etc., from a surface of a substrate, such as a semiconductor wafer, to clean the substrate surface in a semiconductor device manufacturing process. The present invention also relates to a substrate processing apparatus and method for polishing a surface of a substrate into a flat mirror-like surface.
2. Description of the Related Art
It has been confirmed that an organic material present on a surface of a substrate, such as a semiconductor wafer, or a natural oxide film produced on surfaces of copper interconnects formed in the surface of the substrate, can be removed by a method comprising vacuum-evacuating a chamber containing therein the substrate, and then putting the substrate in a formic acid gas atmosphere at a gas pressure of not less than 40 Pa while heating the substrate at a temperature of not less than 130° C. An exemplary processing time is about 0.1 minute when the formic acid gas pressure is 400 Pa and the substrate temperature is 200° C.
In a semiconductor device manufacturing process, a cleaning treatment of a substrate surface is sometimes necessary before or after an atmospheric processing of the substrate, such as chemical mechanical polishing (CMP) or plating, carried out in the air. For example, a process as shown in FIG. 1 is widely practiced in which a lower-level interconnect film of, e.g., copper is formed by plating on a surface of a substrate, and a surface of the lower-level interconnect film is polished and flattened by CMP to form lower-level interconnects. Thereafter, an insulating film is deposited over the substrate surface, and trenches are formed in the insulating film, and then a barrier metal film is formed over the substrate surface. Thereafter, an upper-level interconnect film of, e.g., copper is formed by plating on a surface of the barrier metal film, followed by CMP to polish and flatten a surface of the upper-level interconnect film, thereby forming upper-level interconnects.
In the above process, a natural oxide film can form on surfaces of the lower-level interconnects prior to the formation of the upper-level interconnects, and an organic material, such as benzotriazole (BTA) used in CMP process to prevent corrosion of copper as an interconnect material, can remain on the substrate surface. The natural oxide film and the organic material both have an inhibitory influence on the electrical connection between the upper-level interconnects and the lower-level interconnects.
It is therefore a conventional practice to remove such a natural oxide film formed on surfaces of interconnects by, for example, plasma cleaning.
With the recent progress toward higher integration of semiconductor devices, the circuit interconnects are becoming finer and the distance between adjacent interconnects is becoming smaller. Especially when forming a circuit pattern by optical lithography with a line width of not more than 0.5 μm, a stepper requires a high flatness of imaging surface because of the small depth of focus. A chemical mechanical polishing apparatus for carrying out chemical mechanical polishing (CMP) is known as a means for flattening a surface of such a semiconductor substrate.
A common chemical mechanical polishing (CMP) apparatus includes a polishing table having a polishing pad on an upper surface, and a top ring, and is designed to polish a surface of a substrate (wafer) into a flat mirror-like surface by pressing the substrate on the polishing table by the top ring while supplying an abrasive liquid (slurry) to the surface of the polishing pad.
When flattening a metal film, formed on a surface of a substrate, by chemical mechanical polishing, it is a conventional practice to oxidize the metal film with an oxidizing agent in a slurry and instantaneously convert the oxidized film into an insoluble complex with a chelating agent in the slurry, and polish away the complex, e.g., with abrasive grains in the slurry.
When a metal film such as copper is formed on a surface of a substrate, there is a case where a natural oxide film grows on the metal film by the moisture or oxygen in the air. The formation of a natural oxide film on a metal film hinders complexing of the substrate surface with a chelating agent. A natural oxide film per se is harder to polish than a complex. Accordingly, in chemical mechanical polishing of a substrate surface in which a natural oxide film having a non-uniform thickness is formed, there is often a case where polishing does not progress locally whereby uniform flattening of the substrate surface cannot be achieved.
In order to polish a metal film formed on a substrate after removing a natural oxide film on the metal film so as to achieve uniform flattening of the substrate surface, a method has been proposed in which prior to flattening polishing processing, a natural oxide film is removed from the substrate surface by an oxide film removal processing unit constructed integrally with a flattening processing unit (see, for example, Japanese Patent Laid-Open Publication No. 2005-277396). The patent document discloses as the oxide film removal processing unit a wet processing unit using an acidic chemical solution and a dry processing unit which carries out reduction or etching of an oxide film in a vacuum apparatus.
There is also a report that when a natural oxide film on a copper surface is cleaned off by using formic acid gas and the sample is then left to stand in the air, a natural oxide film having a thickness of about 0.5 nm is produced on the copper surface in 3 minutes and 40 seconds after the cleaning (see Suzaki et al., “Growth of natural oxide film on Cu surface and removal of oxide film with formic acid gas”, Collected Papers on 2005 Autumn Lecture Meeting of Japan Society of Applied Physics, Sep. 7, 2005, vol. 2, p. 705).
The above-described methods for surface treatment of a substrate have the following drawbacks: The method of treating a substrate surface in a vacuum necessitates a vacuum vessel and a vacuum evacuation means, thus requiring the use of a larger-sized processing apparatus. In addition, the method requires the step of vacuum-evacuating a vacuum vessel and the step of returning the vessel to atmospheric pressure, resulting in a considerably long processing time.
With regard to plasma cleaning of a substrate surface to remove a natural oxide film from the substrate surface, damage to a semiconductor device by plasma treatment becomes marked as interconnects become finer. Furthermore, there is presently no effective measure to remove BTA remaining on a substrate surface. A strong demand therefore exists for the development of a method for surface treatment of a substrate, which takes the place of plasma cleaning and can remove BTA from a substrate surface.
When a natural oxide film on a substrate surface is removed by a wet treatment using an acidic chemical solution, a functional device formed in the substrate can be damaged by the treatment. It is therefore desirable not to use such a wet treatment for an oxide film removal of a highly-integrated semiconductor. On the other hand, in the case of a dry treatment to remove a natural oxide film from a substrate surface using a vacuum apparatus, the treatment generally needs the use of three vacuum chambers, i.e., a vacuum treatment chamber and two spare vacuum chambers (load-lock chambers) for carrying in and out a substrate before and after the treatment, and a vacuum pump. A large-sized oxide film removal apparatus for carrying out the dry treatment is thus needed, and it is practically difficult to incorporate such a large apparatus into a chemical mechanical polishing apparatus.